The invention relates to an integrated CMOS circuit comprising a substrate bias voltage generator, which is driven by a clock generator, which also controls further circuits in the integrated circuit.
An integrated CMOS circuit, in which a substrate bias voltage generator is included, is known from European Patent No. 0029681 published on Jan. 28th 1987. Substrate bias voltage generators are used in CMOS circuits in order to avoid the so-called "latch-up" state. If no substrate bias voltage generators are used, this "latch-up" state is very liable to be attained. In a CMOS process with n wells used frequently, both n- and p- type transistors are implemented, as a result of which parasitic thyristors are also formed. If one or more of these thyristors are ignited at junction points in a circuit and at different areas in a substrate, said junction points of the circuit are held at a fixed voltage by these thyristors. This results in that the circuit no longer operates correctly and has passed into a "latch" state. When a substrate bias voltage is produced, it is ensured that it is much more difficult for the thyristors to ignite. In general, a substrate bias voltage generator consists of a capacitive pump controlled by a ring oscillator. The ring oscillator is also used for producing the clock frequency, which is utilized to control the further circuits in the integrated circuits. However, the "latch-up" sensitivity now depends upon the clock frequency because, if a higher clock frequency is used, the pump of the substrate bias voltage generator must also operate more rapidly in order to be able to compensate for the substrate currents increased by the higher clock frequency.
In European Patent Application EP-A-No. 0022870, a semiconductor circuit including first and second bias voltage generators is shown. This circuit generates both fixed and variable substrate bias voltages, but does not include switchable circuitry to activate or deactivate the bias voltage generating circuitry on demand.
Many integrated CMOS circuits, however, have an input terminal for supplying a clock signal having a frquency desired by the user of the circuit because the circuit must fit into a system designed by the user or because the circuit can operate in different modes, in which event different clock frequencies are required for the different modes. An example thereof is a memory circuit serving to store teletext information in television applications. The clock frequency for the memory is then also determined by the 625 or 525 image line system used in television applications. An oscillator for controlling the charge pump of the substrate bias voltage generator is not integrated in such integrated circuits. The charge pump is then controlled by the clock pulses which are externally applied to the circuit. However, when the integrated circuit is tested by the user, a clock frequency is not always supplied. If in this case tests are carried out, no substrate bias voltage is produced. as a result of which an increased "latch-up" sensitivity is obtained. Further, when the circuit is developed, nevertheless one or several parts of the circuit may unexpectedly pass into the "latch-up" state during measuring procedures on the first manufactured silicon wafers with the CMOS circuit, as a result of which substrate currents will flow, which cause the substrate bias voltage to decrease. This results in that even larger parts of the integrated circuit pass into the "latch-up" state, etc. Ultimately, no measurements can be carried out on the integrated circuit.